1. Field of the Invention
The invention herein relates to the use of gases in medical processes, such as magnetic resonance imaging (MRI). More particularly it relates to the recovery and purification of such gases for reuse.
2. Description of the Prior Art
Various techniques of medical imaging have been developed in the last few years, which provide to physicians the ability to make visual images of patients"" internal organs and bodily processes. Such imaging processes have been invaluable for making diagnoses of illnesses and dysfunctions and in giving surgeons the ability to locate and identify internal lesions and tumors before subjecting patients to surgery. Among the techniques widely used is magnetic resonance imaging (MRI).
MRI has been widely used for imaging the brain, heart, kidneys, and spine, since these organs produce relatively strong magnetic resonance (MR) images so that usable images can be obtained. However, other organs, notably the lungs, have not in the past produced such useful MRI images, since magnetic resonance is lower in these organs, particularly in the lungs which are of course hollow and filled with air.
A new technique, called xe2x80x9chyperpolarized noble gas MRIxe2x80x9d, has been developed and is reported in Albert and Balamore, Physics Res. A [Nucl. Instr. and Meth.], 402:441-453 (1998). The technique involves using the magnetic resonance signal from hyperpolarized noble gases Xe129 and He3 to image the lungs or brains of patients who inhaled one of these gases. Images of sufficient quality to study pulmonary disease and to assist in research to elucidate the link between the structure of the lungs and their function have been obtained. The researchers have found that He3 is easier to hyperpolarize than Xe129 and yields a stronger MR signal. On the other hand Xe129 is dissolved much more easily in blood and can pass the blood brain barrier. Consequently it seems likely that He3 will find greater use in MR imaging of lungs while Xe129 imaging will be used more for imaging of reach structures of the brain and studies of cortical brain function.
Many details of the enhanced MR imaging with He3, as well as details of hyperpolarization, have been recently described in Beardsley, xe2x80x9cSeeing the Breath of Life,xe2x80x9d Scientific American, 280(6):33-34 (June, 1999).
While Xe129 constitutes approximately one-fourth of all Xe isotopes, xenon itself is a relatively rare element, being found as only about 40 ppb in air, with Xe129 thus being present as approximately 10 ppb in air. He3 is even more rare, being present as only slightly more than 1 ppm of all helium. Loss of either of these isotopes during or after an MRI procedure therefore is a very serious matter, not only because of the initial cost of the isotope but also, especially in the case of He3, because the vented material can never be recovered. Because there is so little of these materials in the worldxe2x80x94some estimates are that the maximum world amount of He3 is less than 200 kgxe2x80x94enhanced MR imaging using hyperpolarized He3 or Xe129 is unlikely to become widely used unless there are methods for recovery and recycling of major portions, and preferably substantially all, of these isotopes from their use in MRI procedures.
Other gases, such as hydrocarbon or fluorocarbon gases which contain C13 or F19 isotopes, or those using the isotope P31, also find use in various medical processes, which processes may or may not include imaging or hyperpolarization steps. These types of gases also require recovery, usually to avoid environmental air contamination, and when recovered may be advantageously purified and recycled for reuse.
We have now invented a method for providing a pure gas for use in medical procedures in which the gas is contaminated with other gases during the procedure, and then separating the contaminants and recovering and reusing the decontaminated gas. The method is most advantageously used in medical imaging processes, such as magnetic resonance image (MRI), where hyperpolarized image enhancing noble gases, notably He3 or Xe129, are used for image enhancement in brain and lung imaging, and in which the contaminants are normally the exhalant gases from the imaging patient who inhales and exhales the gas as part of the imaging procedure.
(For brevity herein the method will be described in the context of an MRI process and hyperpolarization of He3 for use therein. It will be understood that this is exemplary only, and that the method is also applicable to provision of pure gases for other medical processes or procedures, and for use with other recoverable gases. It will also be understood that such gases need not be hyperpolarized as part of the particular medical process or procedure.)
The method is a closed loop system in which the feed image enhancing gas provision is either from a storage tank of recycled, purified gas or from makeup fresh gas. The feed gas passes through a preliminary purification unit in which any contaminants which have gotten into the feed gas from tankage or transport are removed. The gas (e.g., He3) is then passed to a hyperpolarization unit in which is subjected to conventional hyperpolarization, as by high energy laser beam. The hyperpolarized gas is discharged from the hyperpolarization unit into a container, such as a gas tight bag, in which it is transported to the location where the imaging procedure is to be conducted. Using as an example an MRI procedure, the patient will inhale gas from the bag and the patient will then hold his or her breath for the short time that it takes for an MRI scan of the patient""s lungs to occur. After the scan is completed, the patient will exhale into another larger bag, usually several times to clear as much of the image enhancing gas as reasonably possible from the lungs. After the final exhalation by the patient, the bag of now-contaminated gas will be passed to the recovery and purification system of this invention, along with the original bag containing uninhaled gas, which will have some lesser degree of contamination from having been opened and having had the patient inhale from it. It is anticipated that a major portion of the feed gas will be returned, excepting only those portions of the gas which the patient has failed to exhale, which have been absorbed by the patient""s body, or which have leaked into the ambient atmosphere during the time that either bag was opened for the patient to indicate or exhale. When the bags of contaminated gas are received by the recovery and purification system, the contaminated gas is removed from each bag and passed through a series of drying and purification steps to remove the exhalant or other contaminant gases and separate the residual image enhancing gas.
The feed gas from storage or fresh makeup to the hyperpolarization unit is normally under elevated/superatmospheric pressure, although it is discharged from the hyperpolarization unit at essentially atmospheric pressure so that it can be inhaled and exhaled by the patient with minimal loss to the ambient atmosphere. After the contaminated gas is returned to the system, it is moved onward through the system at vacuum/subatmospheric pressure by means of a vacuum pump. Depending on the type of decontamination units used to remove the exhalant gases, the contaminated gas may be compressed to superatmospheric pressure before being passed through the decontamination units, it may be passed through the decontamination units at subatmospheric pressure with pressurizing compression occurring only on the quantity purified image enhancing gas after decontamination, or compression may take place at some point intermediate in the passage through the various decontamination units. Regardless of which of these options is used, the purified gas ultimately will be returned to gas storage under superatmospheric pressure so that it can be recycled and reused for subsequent image imaging procedures.
Exhalant gases (including liquid vapors) which are removed from the contaminated gas after return from the imaging procedure usually will include oxygen, water vapor, nitrogen, carbon dioxide, argon and perhaps small amounts of other gases such as hydrogen, hydrocarbons and air pollutant gases such as nitrogen oxides or ozone that the patient has breathed from his or her normal environment or which may have entered the gas sample by handling of its container or from components within the system. It is most important to remove substantially all of the oxygen, carbon dioxide and water vapor before reuse or recycle of the image enhancing gas, because both of these gases will poison the hyperpolarization unit. While it is also important to remove the other exhalant gases, the degree of removal is less important because they will dilute the image enhancing gas. However, since they are inert with respect to hyperpolarization, they do not poison the unit but merely the imaging less efficient because the image enhancing gas in the patient""s lungs after inhalation is less concentrated and therefore the resultant image is of lower quality.
It is important in this invention that all gas transport, compression and storage equipment must be designed and maintained so that it is non-contaminating to the image enhancing gas, so that (with the exception of expected contaminants, usually the exhalant gases and vapors from a patient or the airborne materials which may enter when a gas bag is opened for a patient""s inhalation), other gases, vapors, particulates or foreign matter, whether organic or inorganic, do not come into contact with the gas. It is also preferred that conventional equipment, such as pumps and compressors, which include components which include volatile materials, such as lubricants and sealants, into contact with the gas, not be used. Components such as lubricants, sealants and the like emit minute quantities of unsterile volatile materials, which even though emitted only slowly, soon build up to significant quantities in the gas flow stream and render the image enhancing gas unsafe and unusable, unless expensive and additional decontamination capability is added to the process of the invention. In the present invention, therefore, it is preferred to use sterile equipment, i.e. equipment which does not place any unsterile volatile or transferable gas, liquid or particulate matter in contact with the process gas stream. Such equipment is readily available, either as completely non-contaminating equipment (i.e., without any volatile lubricants, etc.) or as food-contract grade equipment. Use of such equipment in the process permits medical imaging processes to use recovered and recycled gases without deterioration of the MR enhancing properties of the gases or of the images which they are being used to obtain, and, most importantly manufacture, without risk to the imaged patient.
While the present application is described with respect to He3 and Xe129, which are currently being used in research and imaging, it is believed that the present invention is applicable to purification of all gases used in medical procedures, especially those isotopic image enhancing gases, whether or not hyperpolarized, since the method of the present invention is not dependent upon the nature of the isotopic gas in its separation of from contaminants such as patient exhalations. Consequently other gaseous isotopes which are known to be useful in medical imaging procedures, as well as those whose utility may be determined in the future, are intended to be included. It is also intended to include all environmentally hazardous gases, such as hydrocarbons or fluorocarbons which incorporate C13 or F19 isotopes or phosphorus gases having the isotope P31, as well as those gaseous isotopes of sufficient scarcity that recovery is considered economically desirable.
In a broad embodiment, therefore, the invention herein is of a method for recovery and purification of a gas used to enhance a medical process, which comprises passing said gas to said medical process imaging and therein using said gas for enhancement of said process, use in said process also causing gaseous or liquid contaminants including water vapor, carbon dioxide, oxygen or nitrogen, to become incorporated into said gas; collecting at least a portion of thus-contaminated gas; decontaminating said contaminated gas of contaminants thus introduced, which comprises at least one of, in any order and depending on the contaminants incorporated, drying said contaminated gas to reduce contaminant water concentration in said contaminated gas to not greater than 10 ppm; contacting said contaminated gas with a carbon dioxide absorbent to reduce carbon dioxide concentration in said contaminated gas to not greater than 10 ppm; contacting said contaminated gas with an oxygen absorbent to reduce oxygen concentration in said contaminated gas to not greater than 1 ppm; and contacting said contaminated gas with a nitrogen getter to reduce nitrogen concentration in said contaminated gas to not greater than 1 ppm; and collecting said gas after such decontamination for recycle to said medical process and subsequent reuse therein. Most commonly the gas comprises an isotope of helium, xenon, carbon, fluorine or phosphorus.
In a more specific embodiment, the invention herein is of such a method wherein said gas is hyperpolarized prior to use in said medical process, and the medical procedure in which hyperpolarized gas is used comprises medical imaging, commonly magnetic resonance imaging (MRI).